1. FIELD OF THE INVENTION
This invention relates to gas generating apparatus for calibrating analyzers or the like.
2. DESCRIPTION OF THE PRIOR ART
U.S. Pat. No. 3,856,204 discloses a device for emitting a gas at a constant rate into a moving fluid medium to produce an accurately known concentration of the gas in the said fluid medium. The gas is held in a cylinder under pressure, in equilibrium with its liquid phase or solely in its gaseous phase, and permeates through a permeate material filling an accurately dimensioned passage through one end of the cylinder.
It also discloses the use of such a device with an apparatus for the analysis of fluid mixtures. By injection of an accurate volume of said gas in an accurate volume of fluid medium, it makes it possible to easily calibrate analysers or the like.
An improvement of such a device is disclosed in U.S. Pat. No. 4,399,943. The device of this latter patent comprises two chambers, one for holding the substance in liquefied form and the other for holding it solely in gaseous form. The substance permeates through a permeable material between the two chambers, and then through another quantity of permeable material positioned at the exit from the second chamber.
In both cases, a constant temperature is maintained around the cylinder in order to maintain a constant rate of emission of the substance contained in the said cylinder.
However, though maintaining a constant temperature around the cylinder, it has been discovered that the rate of emission of the substance through the permeable material appeared not to be constant. Having made several measurements with the same fluid medium with an injection through the same passage of the device of the same gas substance, namely water vapor, the inventors found variability of concentrations when compared to a certified calibrated hygrometer. It thus appeared to them that some improvements must be made in such devices to obtain accurate and reproductible results, in order to be able to calibrate any type of analysers, in any conditions.
It is of course well known that there is a need in the semi-conductor industry to accurately measure the concentration of impurities, such as water vapor, in the gases used during manufacture of integrated circuits. Particularly, there is a need to calibrate analysers, such as hygrometers, accurately in the 0.01 to 1.0 ppm range and this need was not fulfilled up to now using the permeation devices disclosed above. These prior art devices are not sufficiently accurate, on the one hand, and they give non reproducible samples, on the other hand.